Add `Send` and `Sync` traits to the reference
Fixes #36859
found, `.deref()` is called and the compiler continues to search for the method
implementation in the returned type `U`.
+## The `Send` trait
+
+The `Send` trait indicates that a value of this type is safe to send from one
+thread to another.
+
+## The 'Sync' trait
+
+The 'Sync' trait indicates that a value of this type is safe to share between
+multiple threads.
+
# Memory model
A Rust program's memory consists of a static set of *items* and a *heap*.